Corrosion and cracking resistant high manganese austenitic steels containing passivating elements

ABSTRACT

Improved steel compositions and methods of making the same are provided. The present disclosure provides advantageous corrosion and/or cracking resistant steel. More particularly, the present disclosure provides high manganese (Mn) steel compositions having enhanced corrosion and/or cracking resistance, and methods for fabricating high manganese steel compositions having enhanced corrosion and/or cracking resistance. Methods for fabricating high manganese steel compositions (e.g., via passivation) having enhanced corrosion and/or cracking resistance are also provided.

FIELD

The present disclosure relates to improved steel compositions and methods of making the same, and more particularly, to high manganese (Mn) austenitic steel compositions having enhanced corrosion and/or cracking resistance and methods for fabricating high manganese steel compositions having enhanced corrosion and/or cracking resistance (e.g., via passivation). It is a further objective of this disclosure to provide equipment and components fabricated from such austenitic high Mn steels which can have a combination of high strength, improved sweet and sour corrosion resistance, and resistance to environmentally induced cracking including sulfide stress cracking and stress corrosion cracking.

BACKGROUND

Carbon steels are widely used as structural materials in the oil, gas, and petrochemical industries. The upstream sector in these industries is the major user of structural materials, in which the use of carbon steels constitutes about 95% in tonnage of all the structural materials used. In general, carbon steels are strong, tough, weldable, widely available commercially, and are low cost structural materials. However, carbon steels do not possess inherent corrosion resistance, and carbon steels with high strength are also susceptible to environmentally induced cracking such as sulfide-stress-cracking.

The majority of the rest of the 5% tonnage of the structural materials used in the upstream sector of oil, gas, and petrochemical industry are mostly corrosion resistant alloys (“CRA”) for use in harsh and corrosive environments, among which the most widely used are the austenitic stainless steels. In general, austenitic stainless steels provide a combination of excellent corrosion resistance, cracking resistance, oxidation resistance, and good formability and toughness. These stainless steels typically owe their excellent corrosion resistance to high chromium (Cr) alloying, and typically owe their high ductility and toughness to the austenitic phase having face-centered cubic (FCC) atomic crystalline structure that is stabilized by high nickel (Ni) alloying. As an example, a commonly used austenitic stainless steel, 304 SS, has a nominal composition of about 18 wt. % Cr and 8 wt. % Ni. The cost of the austenitic stainless steels is thus more expensive with a cost typically five to six times higher than that of carbon steels, due largely to their high Ni content, which is an expensive alloying element that at times experiences supply shortages. In addition, the austenitic stainless steels exhibit relatively lower strength compared to ferritic carbon steels and ferritic stainless steels, and are generally susceptible to stress corrosion cracking.

There has been growing interest in lower cost and more cost-effective stainless steels, driven by the interest in reducing the materials cost and/or minimizing the influence of wild fluctuation in nickel prices. This has resulted in the development of, for example, the 200 series austenitic Cr—Mn—Ni stainless steels exemplified by Type 201, 202, and 216, in which the expensive Ni alloying is reduced and partially replaced by the manganese (Mn), which is a lower cost austenite stabilizer. In general, these 200 series stainless steels provide superior strength and comparable ductility compared to 304SS. However, most of these Ni-reduced steels show strong increases in yield strength and severe loss of ductility associated with the precipitation of carbides, nitrides, and/or carbon nitrides when exposed to elevated temperatures ranging from about 600° C. to about 900° C.

High alloy Corrosion Resistant Alloy (CRA) pipes and/or tubing for down-hole application is traditionally cold worked or forged to obtain higher yield and tensile strengths required to contain high pressure and support high tension loads from the weight of the pipe/tubing. In order to obtain sufficient high strength to meet the required mechanical strengths for the finished pipe/tubing, the pipe/tubing can be cold deformed either by drawing where a larger hollow is pulled or drawn through a smaller die, reducing the OD and simultaneously reducing the ID over a retained mandrel and then repeat the same process to obtain the required mechanical strengths. Alternatively, the pipe/tube can be cold deformed by pilgering where a hollow pipe/tube is mechanically forged under high pressure through a set of dies substantially reducing the OD and simultaneously reducing the ID over a mandrel to obtain the required mechanical strengths. Selected chemistry of high Mn steels may exhibit dynamic strain aging (DSA) during the cold deformation and/or elevated temperature deformation. DSA can be utilized as an effective way to enhance strength and fracture toughness and more effective than cold deformation by dislocation glide due to more uniform distribution of dislocations and more effective dislocation generation.

Thus, a need exists for improved steel compositions to achieve enhanced corrosion and/or cracking resistance, and methods for fabricating the same. These and other inefficiencies and opportunities for improvement are addressed and/or overcome by the systems and methods of the present disclosure.

SUMMARY

The present disclosure provides advantageous steel compositions. More particularly, the present disclosure provides improved high manganese (Mn) steel compositions having enhanced corrosion/cracking resistance, and related methods for fabricating steel compositions having enhanced corrosion/cracking resistance.

In general, the present disclosure provides for ferrous austenitic stainless steel compositions having relatively lower cost compared to conventional austenitic stainless steels (e.g., 304SS). The advantageous ferrous austenitic steel compositions of the present disclosure can be utilized for oil/gas and/or petrochemical applications wherein improved corrosion resistance (e.g. for sweet and sour corrosion), cracking resistance, and/or cost-effectiveness are desired/required.

In exemplary embodiments, the microstructure of the ferrous based steel compositions of the present disclosure include a predominantly austenite phase having a face-centered-cubic (FCC) atomic crystalline structure. In certain embodiments, the exemplary ferrous based steel compositions include a high amount of manganese (e.g., greater than or equal to about 8 wt. %), and passive film forming elements including but not limited to chromium (e.g., greater than or equal to about 11 wt. %), and/or aluminum (e.g., greater than or equal to about 1 wt. %), and/or Titanium, and/or silicon (e.g., greater than or equal to about 0.5 wt. %) and their combinations thereof. The exemplary ferrous based steel compositions may further contain a high amount of interstitial alloying elements such as, for example, carbon (e.g. greater than or equal to about 0.1 wt. %) and/or nitrogen (e.g. greater than or equal to about 0.1 wt. %).

As such, the present disclosure provides cost-effective ferrous austenitic stainless steel compositions (e.g., for the oil, gas, petrochemical industry), and methods for fabricating cost-effective ferrous austenitic stainless steel compositions. The exemplary steel compositions have advantages over many commonly used austenitic stainless steels, such as 304SS. For example, some of these advantages include, without limitation, one or more of the following: (i) lower materials cost by replacing or reducing Ni with Mn, C, N, and/or combinations thereof, (ii) higher strength due to higher nitrogen and carbon contents facilitated by high Mn alloying, and/or due to cold deformation and/or elevated temperature deformation, (iii) improved corrosion resistance due to high alloying addition of passive film forming elements (e.g. Cr, Al, Ti, and/or Si), (iv) improved pitting corrosion resistance due to higher nitrogen alloying facilitated by Mn alloying, and/or (v) maintaining toughness and cracking resistance derived from the austenite phase.

Another aspect of the present disclosure is to fabricate equipment at least in part out of the exemplary ferrous austenitic stainless steels. Some exemplary uses/applications of the steel compositions of present disclosure include, without limitation, use in oil, gas, and petrochemical equipment/systems, such as for reactor vessels, pipes, casings, packers, couplings, sucker rods, seals, wires, cables, bottom hole assemblies, tubing, valves, compressors, pumps, bearings, extruder barrels, molding dies, and combinations thereof.

Exemplary Methods for Fabrication

The present disclosure provides for a method for fabricating a ferrous based component including: a) providing a composition having from about 8 to about 30 weight % manganese, from about 11 to about 30 weight % chromium, and the balance iron; b) melting the composition in a controlled environment to produce a liquid alloy steel composition; c) cooling the liquid alloy steel composition to form an alloy steel composition; d) hot deforming the alloy steel composition; e) re-heating the alloy steel composition for a pre-determined time period; and f) cooling the alloy steel composition.

The present disclosure also provides for a method for fabricating a ferrous based component wherein the composition further includes one or more alloying elements selected from the group consisting of carbon, nitrogen, aluminum, silicon, nickel, cobalt, molybdenum, niobium, copper, titanium, vanadium, tungsten, boron, zirconium, hafnium and combinations thereof.

Any combination or permutation of embodiments is envisioned. Additional advantageous steps, features, functions and applications of the disclosed systems and methods of the present disclosure will be apparent from the description which follows, particularly when read in conjunction with the appended figures. All references listed in this disclosure are hereby incorporated by reference in their entireties.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and aspects of embodiments are described below with reference to the accompanying drawings, in which the elements in the drawings are not necessarily depicted to scale.

Exemplary embodiments of the present disclosure are further described with reference to the appended figures. It is to be noted that the various steps, features and combinations of steps/features described below and illustrated in the figures can be arranged and organized differently to result in embodiments which are still within the spirit and scope of the present disclosure. To assist those of ordinary skill in the art in making and using the disclosed systems, assemblies and methods, reference is made to the appended figures, wherein:

FIG. 1 is an exemplary diagram of the phase stability and deformation mechanism of high Mn steels as a function of alloy chemistry and temperature;

FIG. 2 depicts a Fe—Cr—Mn—Ni phase diagram for exemplary ferrous austenitic stainless steel compositions of the present disclosure;

FIGS. 3A-B depict scanning electron micrographs showing the typical microstructure of exemplary steels of the present disclosure: (a) in as-rolled plate with the secondary Cr-rich carbides in the predominantly austenite matrix; and (b) in solutionized heat treatment with the predominantly austenite microstructure;

FIG. 4A depicts polarization curves of [20 wt. % Mn, 18 wt. % Cr, 0.6 wt. % C, 0.4 wt. % N, and the balance Fe] alloy steel in both the as-rolled and in the solutionized heat treated state, and the commercial 304 SS comparison sample; and

FIG. 4B depicts the corrosion rates of [20 wt. % Mn, 18 wt. % Cr, 0.6 wt. % C, 0.4 wt. % N, and the balance Fe] alloy steel in both the as-rolled and in the solutionized heat treated state, and the commercial 304 SS and carbon steel comparison samples.

DETAILED DESCRIPTION

The exemplary embodiments disclosed herein are illustrative of advantageous steel compositions, and systems of the present disclosure and methods/techniques thereof. It should be understood, however, that the disclosed embodiments are merely exemplary of the present disclosure, which may be embodied in various forms. Therefore, details disclosed herein with reference to exemplary steel compositions/fabrication methods and associated processes/techniques of assembly and use are not to be interpreted as limiting, but merely as the basis for teaching one skilled in the art how to make and use the advantageous steel compositions of the present disclosure. Drawing figures are not necessarily to scale and in certain views, parts may have been exaggerated for purposes of clarity.

All numerical values within the detailed description and the claims herein are modified by “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the present disclosure. Ranges from any lower limit to any upper limit are contemplated. The upper and lower limits of these smaller ranges which may independently be included in the smaller ranges is also encompassed within the present disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the present disclosure.

Although methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.

It must be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this present disclosure belongs. The terminology used in the description of the present disclosure herein is for describing particular embodiments only and is not intended to be limiting of the present disclosure. All publications, patent applications, patents, figures and other references mentioned herein are expressly incorporated by reference in their entirety.

DEFINITIONS

CRA: Corrosion resistant alloys, can mean, but is in no way limited to, a material specially formulated to have good corrosion resistance for fabricating equipment to be used in a corrosive environment. Corrosion-resistant alloys may be formulated for a wide range of harsh and corrosive conditions.

Ductility: can mean, but is in no way limited to, a measure of a material's ability to undergo appreciable plastic deformation before fracture; it may be expressed as percent elongation (% EL) or percent area reduction (% AR).

Corrosion Resistance: can mean, but is in no way limited to, a material's inherent resistance to deterioration caused by exposure to a reactive or corrosive environment.

Toughness: can mean, but is in no way limited to, resistance to crack initiation and propagation.

Stress Corrosion Cracking (SCC): can mean, but is in no way limited to, the cracking of materials due to simultaneous action of stress and a reactive and corrosive environment.

Sulfide Stress Cracking (SSC): can mean, but is in no way limited to, cracking of materials due to exposure to fluids containing hydrogen sulfide (e.g., H₂S).

Yield Strength: can mean, but is in no way limited to, the ability of a material to bear load without deformation.

Cooling rate: can mean, but is in no way limited to, the rate of cooling of a piece of material, which in general is measured at the center, or substantially at the center, of a piece of material.

Austenite: can mean, but is in no way limited to, a metallurgical phase in steels that has a face-centered cubic (FCC) atomic crystalline structure.

Martensite: can mean, but is in no way limited to, a metallurgical phase in steels that can be, but not limited to, formed by diffusionless phase transformation in which the parent (typically austenite) and product phases have a specific orientation relationship.

ε(epsilon)-martensite: can mean, but is in no way limited to, a specific form of martensite having hexagonal close packed atomic crystalline structure which forms upon cooling or straining of austenite phase. ε-martensite typically forms on close packed (111) planes of austenite phase and is similar to deformation twins or stacking fault clusters in morphology.

α′(alpha prime)-martensite: can mean, but is in no way limited to, a specific form of martensite having body-centered cubic (BCC) or body-centered tetragonal (BCT) atomic crystalline structure which forms upon cooling or straining of austenite phase; α′-martensite typically forms as platelets.

M_(s) temperature: can mean, but is in no way limited to, the temperature at which transformation of austenite to martensite starts during cooling.

M_(f) temperature: can mean, but is in no way limited to, the temperature at which transformation of austenite to martensite finishes during cooling.

M_(d) temperature: can mean, but is in no way limited to, the highest temperature at which a designated amount of martensite forms under defined deformation conditions. M_(d) temperature is typically used to characterize the austenite phase stability upon deformation.

Carbide: can mean, but is in no way limited to, a compound of iron/metal and carbon.

Cementite: can mean, but is in no way limited to, a compound of iron/metal and carbon having approximate chemical formula of M₃C with orthorhombic atomic crystalline structure.

Pearlite: can mean, but is in no way limited to, typically a lamellar mixture of two-phases, made up of alternate layers of ferrite and cementite (M₃C).

Grain: can mean, but is in no way limited to, an individual crystal in a polycrystalline material.

Grain boundary: can mean, but is in no way limited to, a narrow zone in a metal corresponding to the transition from one crystallographic orientation to another, thus separating one grain from another.

Quenching: can mean, but is in no way limited to, accelerated cooling by any means whereby a fluid selected for its tendency to increase the cooling rate of the steel is utilized, as opposed to air cooling.

Accelerated cooling start temperature (ACST): can mean, but is in no way limited to, the temperature reached at the surface of the plate, when quenching is initiated.

Accelerated cooling finish temperature (ACFT): can mean, but is in no way limited to, the highest, or substantially the highest, temperature reached at the surface of the plate, after quenching is stopped, because of heat transmitted from the mid-thickness of the plate.

Slab: a piece of steel having any dimensions.

Recrystallization: the formation of a new, strain-free grain structure grains from cold-worked metal accomplished by heating through a critical temperature.

T_(nr) temperature: the temperature below which austenite does not recrystallize.

Dynamic strain aging (DSA): the phenomenon related to the increase in yield strength and hardness of the material upon straining due to interaction of solute atoms (e.g., carbon, and/or nitrogen) with dislocations or pinning of dislocations by solute atoms to increase the necessary stress for the dislocation movement.

The present disclosure provides advantageous steel compositions (e.g., having enhanced corrosion/cracking resistance). More particularly, the present disclosure provides improved high manganese (Mn) steel compositions having enhanced corrosion/cracking resistance, and methods for fabricating high manganese steel compositions having enhanced corrosion/cracking resistance.

In exemplary embodiments, the present disclosure provides for ferrous austenitic stainless steel compositions having relatively lower cost compared to conventional austenitic stainless steels (e.g., 304SS). As such, the advantageous ferrous austenitic steel compositions of the present disclosure can be utilized for oil/gas/petrochemical applications where improved corrosion resistance (e.g. for sweet and sour corrosion), cracking resistance, and/or cost-effectiveness are desired/required.

In certain embodiments, the microstructure of the exemplary ferrous based steel compositions include a predominantly austenite phase having a FCC atomic crystalline structure. In some embodiments, the exemplary steel compositions include a high amount of Manganese (e.g., >8 wt. %), and passive film forming elements including but not limited to Cr (e.g., >11 wt. %), and/or Al (e.g., >1 wt. %), and/or Si (e.g., >0.5 wt. %), and/or Ti (e.g., >1 wt. %), and their combinations thereof. The exemplary steel compositions may contain a high amount of interstitial alloying elements such as, for example, carbon (e.g., >0.1 wt. %) and/or nitrogen (e.g., >0.1 wt. %).

As such, the present disclosure provides cost-effective ferrous austenitic stainless steel compositions (e.g., for the oil, gas, petrochemical industry), and methods for fabricating cost-effective ferrous austenitic stainless steel compositions. The exemplary steel compositions have advantages over many commonly used austenitic stainless steels (e.g., 304SS). Some of these advantages can include, without limitation, lower materials cost by replacing or reducing Ni with Mn, C, N, and/or combinations thereof; higher strength due to higher nitrogen and carbon contents facilitated by high Mn alloying and/or due to pre-deformation; improved corrosion resistance due to high alloying addition of passive film forming elements (e.g. Cr, Al, and/or Si); improved pitting corrosion resistance due to higher nitrogen alloying facilitated by Mn alloying; and/or maintaining toughness and cracking resistance derived from the austenite phase.

The steel compositions as described or embraced by the present disclosure may be advantageously utilized in many systems/applications (e.g., oil, gas, and/or petrochemical equipment/systems, such as for reactor vessels, pipes, casings, packers, couplings, sucker rods, seals, wires, cables, bottom hole assemblies, tubing, valves, compressors, pumps, bearings, extruder barrels, molding dies, etc.), particularly where corrosion/cracking resistances are important/desired.

In exemplary embodiments, the present disclosure provides for ferrous based components/compositions containing manganese. In certain embodiments, the components/compositions include from about 5 to about 40 weight % manganese, from about 0.01 to about 3.0 weight % carbon, and the balance iron. The components/compositions can also include one or more alloying elements, such as, without limitation, from about 0 to 30 weight % chromium, from about 1 to 15 weight % aluminum, from about 0.01 to 10 weight % silicon, and their combinations thereof. The components/compositions can also include one or more additional alloying elements, such as, without limitation, nickel, cobalt, tungsten, molybdenum, niobium, copper, titanium, vanadium, nitrogen, boron and combinations thereof. Exemplary ferrous based components/compositions containing manganese (and optionally other alloying elements) are described and disclosed in U.S. Patent Pub. No. 2012/0160363, the entire contents of which is hereby incorporated by reference in its entirety.

Component Composition

In some embodiments and as noted above, the ferrous based compositions can include from about 5 to about 40 weight % manganese (preferably from about 8 to about 30 weight % manganese), from about 0.01 to about 3.0 weight % carbon (preferably from about 0.1 to about 1.50 weight % carbon), and the balance iron.

The components/compositions can also include one or more alloying elements, such as, without limitation, from about 0 to about 30 wt. % chromium (more preferably from 11 to 30 wt. %), from about 1 to about 15 wt. % aluminum (more preferably from 2 to 10 wt. %), from about 0.01 to about 10 wt. % silicon (more preferably from 1.5 to 5 wt. %), from about 0.01 to about 10 wt. % titanium (more preferably from 1.5 to 5 wt. %), and their combination thereof.

The components/compositions can also include one or more alloying elements, such as, without limitation, silicon, nickel, cobalt, molybdenum, niobium, copper, titanium, vanadium, tungsten, nitrogen, boron, zirconium, hafnium and combinations thereof. Weight percentages are based upon the weight of the total component/composition.

Nickel may be included in the component from about 0 to about 20 wt. %. Cobalt may be included in the component from about 0 to about 20 wt. %. Molybdenum may be included in the component from about 0 to about 10 wt. % (more preferably from 0.3 to 5 weight %). Niobium, copper, titanium, tungsten and/or vanadium can each be included in the component from about 0.2 to about 10 wt. % (more preferably from 0.3 to 5 weight %). Nitrogen can be included in the component from about 0.01 to about 3.0 wt. % (more preferably from 0.1 to 1.5 weight %). Boron can be included in the component from about 0 to about 0.1 wt. % (more preferably from 0.001 to 0.1 weight %).

The ferrous based components/compositions containing manganese may also include another alloying element selected from the group consisting of zirconium, hafnium, and combinations thereof. Each of these other alloying elements may be included in the component/composition in ranges from about 0 to about 6 wt. % (more preferably from 0.2 to 5 wt. %).

Alloying for Strain Induced Transformations

In general, the mechanical properties of the high Mn steels of the present disclosure are dependent on the characteristics of strain-induced transformation, which is typically controlled by the chemical composition of the steels and/or the processing temperatures. Unlike conventional carbon steels, high Mn steels include a metastable austenite phase with a face centered cubic (FCC) atomic crystalline structure at ambient temperature (e.g., 18-25° C.). Upon straining, the metastable austenite phase can transform into several other phases through strain-induced transformation. More particularly, the austenite phase could transform into microtwins (FCC twin aligned with matrix), ε-martensite (hexagonal lattice), and α′-martensite (body centered tetragonal lattice), depending on steel chemistry and/or temperature. These transformation products could impart a range of unique properties to high Mn steels. For example, fine microtwins effectively segment primary grains and act as strong obstacles for dislocation motion. This leads to effective grain refinement which results in an excellent combination of high ultimate strength and ductility.

Steel chemical composition and temperature are known to be the primary factors controlling the strain-induced phase transformation pathways as shown in FIG. 1. In general, high Mn steels can be divided into four groups depending on the stability of austenite phase upon straining and temperature, e.g., highly stable (A), mildly metastable (B), moderately metastable (C) and highly metastable (D) Mn steel. The metastability of these phases is affected by both temperature and strain. These steels would tend to be more metastable (e.g., higher tendency to transform) at higher temperatures and higher strains.

FIG. 1 is an exemplary diagram of the phase stability and deformation mechanism of high Mn steels as a function of alloy chemistry and temperature. The letters (A, B, C, and D) indicate the various transformation routes during deformation. In this diagram, steel A would deform by dislocation slip but would not transform to a different phase (similar to other metals and alloys), while steels B-D would transform during deformation.

Steels in area A, which has stable austenite and deforms primarily by dislocation slip upon mechanical straining, can be produced with high Mn content (e.g., greater than or equal to about 25 wt. %). In general, steels with a fully stabilized austenitic structure show lower mechanical strength but remain tough at cryogenic temperatures, provide low magnetic permeability and are highly resistant to hydrogen embrittlement.

Steels in area B, which has mildly metastable austenite phase, can be produced with intermediate manganese content (e.g., from about 15 to about 25 wt. % Mn, and about 0.6 wt. % C). These steels form twins during deformation. In general, in this type of steels, a large amount of plastic elongation can be achieved by the formation of extensive deformation twins along with dislocation slip, a phenomenon known as Twinning-Induced Plasticity (TWIP). Twinning causes a high rate of work hardening as the microstructure is effectively refined, as the twin boundaries act like grain boundaries and strengthen the steel due to the dynamic Hall-Petch effect. TWIP steels combine extremely high tensile strength (e.g., greater than 150 ksi) with extremely high uniform elongation (e.g., greater than 95%), rendering them highly attractive for many applications.

Steels in area C, which has moderately metastable austenite phase, can be produced with lower manganese content (e.g., from about 10 to about 18 wt. % Mn). In general, this type of steels can transform into ε-martensite (hexagonal lattice) upon straining. Upon mechanical straining, these steels would deform predominantly by the formation of ε-martensite, along with dislocation slip and/or mechanical twinning.

Steels in area D, which has highly metastable austenite phase, can be produced with even lower manganese content (e.g., from about 4 to about 12 wt. % Mn). In general, this type of steels will transform to a strong phase having body-centered cubic atomic crystalline structure (referred to as α′-martensite) upon deformation. This strong phase can also provide resistance to wear and erosion.

Thus, the chemistry of the high Mn steels can be tailored to provide a range of properties by controlling their transformation during deformation.

Other Alloying Concepts in High Mn Steels

Alloying elements in high Mn steels determine the stability of the austenite phase and strain-induced transformation pathways. In general, manganese is the main alloying element in high Mn steels, and it is important in stabilizing the austenitic structure both during cooling and deformation. In the Fe—Mn binary system, with increasing Mn content to stabilize austenite phase, the strain induced phase transformation pathway changes from α′-martensite to ε-martensite and then to micro-twinning.

Carbon is also an effective austenite stabilizer and the carbon solubility is high in the austenite phase. Therefore, carbon alloying can also be used to stabilize the austenite phase during cooling from the melt and during plastic deformation. Carbon also increases the strength of the austenite matrix by solid solution strengthening. As noted, the carbon in the components/compositions of the present disclosure may range from about 0.01 to about 3.0 wt. %.

Aluminum is a ferrite stabilizer and thus destabilizes austenite phase during cooling. The addition of aluminum to high Mn steels, however, stabilizes the austenite phase against strain-induced phase transformation during deformation. Furthermore, it strengthens the austenite by solid solution strengthening. The addition of aluminum also promotes the formation of passive film, which increase the corrosion resistance of the high Mn containing ferrous based components disclosed herein. The aluminum in the components/compositions of the present disclosure may range from about 0.0 to about 15 wt. %.

Silicon is a ferrite stabilizer and sustains the α′-martensite transformation while promoting ε-martensite formation upon deformation at ambient temperature. Due to solid solution strengthening, addition of Si strengthens the austenite phase by about 50 MPa per 1 wt. % addition of Si. The silicon in the components/compositions of the present disclosure may range from about 0.01 to about 10 wt. %.

Titanium is ferrite stabilizer and often alloyed to stabilize carbon by forming titanium carbides. The stabilization of carbon by titanium carbide formation prevents the formation of chrome carbides that could affect the formation of the “passive” layer. The titanium in the components/compositions of the present disclosure may range from about 0.01 to about 10 wt. %.

Chromium is a ferrite stabilizer and thus enhances the formation of ferrite phase during cooling. The addition of chromium also promotes the formation of passive film, which increase the corrosion resistance of the high Mn containing ferrous based components disclosed herein. Furthermore, the addition of Cr to the Fe—Mn alloy system reduces the thermal expansion coefficient. The chromium in the components/compositions of the present disclosure may range from about 0.5 to about 30 wt. % of the total component.

Copper is austenite stabilizer and enhances strength by solid solution hardening. The copper in the components/compositions of the present disclosure may range from about 0.5 to about 10 wt. % of the total component.

Sulfur is used to improve machinability where the sulfide inclusion (e.g., FeS, MnS, and/or combinations thereof act as chip breakers). Sulfur is generally kept to low levels below 0.02 wt. % as it can reduce the resistance to pitting corrosion.

Based on the understanding of these alloying element effects, suitable steel chemistries can be designed for specific applications. Some criteria for the design of high Mn steels can be the critical martensite transformation temperatures, e.g., M_(s) and M_(εs). M_(s) is a critical temperature below which austenite to α′-martensite transformation occurs, and M_(εs) is a critical temperature below which austenite to ε-martensite transformation takes place.

Exemplary Compositions and Microstructures

Exemplary steel composition concepts of the present disclosure are shown in FIG. 2. In exemplary embodiments, the steel compositions use Cr alloying to promote passive film formation for corrosion resistance, and use Mn and/or Ni alloying to stabilize the austenite (a.k.a. γ) phase for toughness and/or cracking resistance. Cr, however, is a ferrite (a.k.a. α) stabilizer. Hence, as Cr alloying increases, larger amount of Mn and/or Ni alloying addition may be required to stabilize the austenite phase.

The following embodiments below describe some various alloying additions of the ferrous austenitic stainless steel compositions provided by the present disclosure.

In exemplary embodiments, the exemplary steel compositions obtain their predominantly austenitic crystalline structure by replacing the more expensive Ni alloying in conventional austenitic stainless steels with the lower cost Mn (e.g., from about 8 to about 30 wt. % Mn). This enables the reduction of cost, but still achieves the austenite crystalline structure for high ductility and toughness.

In exemplary embodiments, the steel compositions can contain sufficient amounts of Cr (e.g., from about 11 to about 30 wt. %). The Cr alloying addition provides corrosion resistance, including sweet and sour corrosion resistance.

In certain embodiments, the exemplary steel compositions can include high carbon and/or nitrogen alloying (e.g., where the carbon content ranges from about 0.1 to about 1.5 wt. %, and/or the nitrogen content ranges from about 0.1 to about 1.5 wt. %). As noted above, the exemplary ferrous austenitic stainless steel composition enhances the nitrogen solubility in the melt and in the steels by manganese alloying addition.

In certain embodiments, the exemplary ferrous austenitic stainless steel compositions can include Al alloying (e.g., Al addition ranging from about 1 to about 15 wt. %, preferably from about 2 to about 10 wt. %).

In some embodiments, the exemplary ferrous austenitic stainless steel compositions can include Si (e.g., where the Si alloying ranges from about 0.5 to about 10 wt. %, preferably ranging from about 1.5 to about 5 wt. %).

In some embodiments, the exemplary ferrous austenitic stainless steel compositions can include one or more of niobium (Nb), titanium (Ti), vanadium (V), tungsten (W), and molybdenum (e.g., where the total content of these elements each ranges from about 0.3 to about 5 wt. %, respectively).

In exemplary embodiments, the ferrous austenitic stainless steel composition include predominantly austenite and minor phases of carbides, nitrides, and/or carbonitrides, and combinations thereof. The carbon and nitrogen contents of the exemplary steel compositions are selected to provide a range of strength levels (e.g., ranging from about 50 ksi to about 120 ksi in as-fabricated condition without cold deformation). The exemplary steel compositions can be cold deformed to achieve even higher yield strength, for instance in excess of 100 ksi.

Applications

Another aspect of the present disclosure is to fabricate equipment or the like in part or in entirety out of the exemplary ferrous austenitic stainless steels. Such exemplary equipment can include, without limitation, oil/gas and/or petrochemical equipment, wherein the oil/gas and/or petrochemical equipment may include reactor vessels, pipes, casings, packers, couplings, sucker rods, seals, wires, cables, bottom hole assemblies, tubing, valves, compressors, pumps, bearings, extruder barrels, molding dies, and combinations thereof.

Processing

The steel compositions/components of the present disclosure can be fabricated or manufactured by various processing techniques including, but not limited to, various exemplary thermo-mechanical controlled processing (“TMCP”) techniques, steps or methods.

The present disclosure further provides exemplary process steps for fabricating/producing the exemplary ferrous austenitic stainless steels.

First, the exemplary ferrous steel constituents/components can be melted in a controlled environment (e.g., at about 1600° C. for about 30 minutes, and wherein evaporation losses of N and/or Mn are controlled) to produce liquid alloy steel.

Next, the liquid alloy steel may be cooled to about room temperature or or to a suitable low temperature to form the predominantly austenitic structure of the alloy steel (e.g., in the form of casting ingots, or continuously cast as slabs, billets and/or blooms). For example, the liquid alloy steel can be ingot or continuously casted into a mold (e.g., a water-cooled copper mold) to form casting ingots/slabs, while suppressing Mn segregation.

Next, the alloy steel ingots/slabs can then be hot deformed (e.g., at or above about 800° C.) to control grain size, and/or to control the shape of the alloy steel to bars, sheets, tubes, etc.

In some embodiments, the alloy steel (e.g., alloy steel bars/sheets) can then be optionally reheated or subject to a solutionizing treatment (e.g., at or above about 1000° C. for about one hour) to substantially dissolve the secondary phases formed in the previous hot deformation step.

Next, the reheated/solutionized alloy steel can be cooled (e.g., in a water quench bath) rapidly (e.g., at least about 10° C. per second) to a suitable cooling stop temperature (e.g., typically below about 300° C., or to about room temperature) to form the predominantly austenitic structure of the alloy steel.

In some embodiments, the alloy steel (e.g., alloy steel bars/sheets) can be optionally cold deformed (e.g. rolling or forging) to induce strain induced transformation to further increase the strength of the alloy steel.

The present disclosure will be further described with respect to the following examples; however, the scope of the disclosure is not limited thereby. The following examples illustrate improved systems and methods for fabricating or producing improved steel compositions (e.g., improved high Mn steel compositions having enhanced corrosion and/or cracking resistance).

EXAMPLES Example 1

An exemplary alloy of the present disclosure was prepared by vacuum induction melting. Prior to melting, the nominal chemistry of the steel composition was about 20 wt. % Mn, about 18 wt. % Cr, about 0.6 wt. % C, about 0.4 wt. % N, and the balance Fe. The ferrous steel composition was fabricated by vacuum induction melting to produce liquid alloy steel in a controlled environment wherein evaporation losses of N and Mn were controlled.

The liquid alloyed steel was then poured into a water-cooled Cu (copper) mold to form uniform casting ingots (and while suppressing Mn segregation) and then cooled to about ambient temperature.

The ingots were then hot deformed at temperatures above about 800° C. to control the grain size and to shape the alloy steel into cylindrical bars.

Some of the bars were then re-heated/solutionized heat treated for about 1 hour at about 1200° C. to substantially dissolve the secondary phase formed during the hot deformation. The solutionized heat-treated samples were then cooled rapidly (e.g., at least about 10° C. per second) in a water quench bath to about room temperature to form the predominantly austenitic structure of the alloy steel.

The as-rolled samples (i.e., the non-solutionized heat-treated samples) exhibited secondary Cr-rich carbides as shown in FIG. 3A. This microstructure should be avoided to ensure improved corrosion resistance.

As shown in FIG. 3B, a predominantly austenitic structure was achieved in the solutionized heat-treated samples, where these samples were solutionized heat-treated for about 1 hour at about 1200° C., followed by cooling in water.

Electrochemical polarization curves were obtained on the exemplary steels, as well as the commercial 304 grade stainless steel (19 wt. % Cr, 8 wt. % Ni, balance Fe). The test aqueous solutions with 50.2 g/L NaHCO₃, and 3 wt. % NaCl were made from analytical grade reagents and deionized water. The test aqueous solution was purged with about 1 bar carbon dioxide (CO₂) gas at a rate of about 100 cc/minute during the testing. The surfaces of the steel samples were ground with 600 grit silicon carbon paper prior to the testing. FIG. 4A illustrates the polarization curves from the exemplary steels of the present disclosure and the commercial 304 SS at about 25° C. The solution treated inventive steels showed similar corrosion resistance compared to 304 SS, as shown in FIG. 4B.

Whereas the disclosure has been described principally in connection with steel compositions for use in components for the oil, gas and/or petrochemical industry/systems/applications, such descriptions have been utilized only for purposes of disclosure and are not intended as limiting the disclosure. To the contrary, it is to be recognized that the disclosed steel compositions are capable of use in a wide variety of applications, systems, operations and/or industries.

Although the systems and methods of the present disclosure have been described with reference to exemplary embodiments thereof, the present disclosure is not limited to such exemplary embodiments and/or implementations. Rather, the systems and methods of the present disclosure are susceptible to many implementations and applications, as will be readily apparent to persons skilled in the art from the disclosure hereof. The present disclosure expressly encompasses such modifications, enhancements and/or variations of the disclosed embodiments. Since many changes could be made in the above construction and many widely different embodiments of this disclosure could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense. Additional modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the disclosure. 

What is claimed is:
 1. A method for fabricating a ferrous based component comprising the steps of: a. providing a composition having from 8 to 30 weight % manganese, from 11 to 30 weight % chromium, and the balance iron; b. melting the composition in a controlled environment to produce a liquid alloy steel composition; c. cooling the liquid alloy steel composition to form an alloy steel composition; d. hot deforming the alloy steel composition; e. re-heating the alloy steel composition for a pre-determined time period; f. cooling the alloy steel composition; and g. Cold deforming the alloy steel composition.
 2. The method for fabricating a ferrous based component of claim 1, wherein step b) includes melting the composition at 1600° C. for 30 minutes.
 3. The method for fabricating a ferrous based component of claim 1, wherein step c) includes cooling the liquid alloy steel composition to ambient temperature to form the alloy steel composition.
 4. The method for fabricating a ferrous based component of claim 1, wherein step d) includes hot deforming the alloy steel composition at or above 800° C.
 5. The method for fabricating a ferrous based component of claim 1, wherein step e) includes re-heating the alloy steel composition at or above 1000° C. for one hour.
 6. The method for fabricating a ferrous based component of claim 1, wherein step e) includes re-heating the alloy steel composition at or above 1200° C. for one hour.
 7. The method for fabricating a ferrous based component of claim 1, wherein step f) includes cooling the alloy steel composition to below 300° C.
 8. The method for fabricating a ferrous based component of claim 7, wherein step f) includes cooling the alloy steel composition at least 10° C. per second until the alloy steel composition is below 300° C.
 9. The method for fabricating a ferrous based component of claim 1, wherein the composition further includes one or more alloying elements selected from the group consisting of carbon, aluminum, silicon, nickel, cobalt, molybdenum, niobium, copper, titanium, vanadium, tungsten, nitrogen, boron, zirconium, hafnium and combinations thereof.
 10. The method for fabricating a ferrous based component of claim 9; a. wherein each of the nickel or cobalt ranges from 0.5 to 20 weight % of the total composition; b. wherein the aluminum ranges from 0.1 to 15 weight % of the total composition; c. wherein each of the molybdenum, niobium, copper, titanium, tungsten or vanadium ranges from 0.2 to 10 weight % of the total composition; d. wherein the silicon ranges from 0.01 to 10 weight % of the total composition; e. wherein the nitrogen ranges from 0.01 to 3.0 weight % of the total composition; f. wherein the boron ranges from 0.001 to 0.1 weight % of the total composition; and g. wherein each of the zirconium or hafnium ranges from 0.2 to 6 weight % of the total composition.
 11. The method for fabricating a ferrous based component of claim 1, wherein the composition comprises of: a. from 8 to 30 weight % manganese, from 11 to 30 weight % chromium; b. one or more of the alloying additions including, from 0.10 to 1.5 weight % carbon, from 0.10 to 1.5 weight % nitrogen, and their combination thereof; c. from 0.5 to 10 weight % silicon, from 1.0 to 15 weight % aluminum, and their combination thereof; and d. the balance iron.
 12. A ferrous based component fabricated according to the steps comprising: a. providing a composition having from 8 to 30 weight % manganese, from 11 to 30 weight % chromium, and the balance iron; b. melting the composition in a controlled environment to produce a liquid alloy steel composition; c. cooling the liquid alloy steel composition to form an alloy steel composition; d. hot deforming the alloy steel composition; e. re-heating the alloy steel composition for a pre-determined time period; f. cooling the alloy steel composition; and g. cold deforming the alloy steel composition.
 13. The ferrous based component of claim 12, wherein step b) includes melting the composition at 1600° C. for 30 minutes.
 14. The ferrous based component of claim 12, wherein step c) includes cooling the liquid alloy steel composition to ambient temperature to form the alloy steel composition.
 15. The ferrous based component of claim 12, wherein step d) includes hot deforming the alloy steel composition at or above 800° C.
 16. The ferrous based component of claim 12, wherein step e) includes re-heating the alloy steel composition at or above 1000° C. for one hour.
 17. The ferrous based component of claim 12, wherein step e) includes re-heating the alloy steel composition at or above 1200° C. for one hour.
 18. The ferrous based component of claim 12, wherein step f) includes cooling the alloy steel composition to below 300° C.
 19. The ferrous based component of claim 18, wherein step f) includes cooling the alloy steel composition at least 10° C. per second until the alloy steel composition is below 300° C.
 20. The ferrous based component of claim 12, wherein the composition further includes one or more alloying elements selected from the group consisting of carbon, aluminum, silicon, nickel, cobalt, molybdenum, niobium, copper, titanium, vanadium, tungsten, nitrogen, boron, zirconium, hafnium and combinations thereof.
 21. The ferrous based component of claim 20: a. wherein each of the nickel or cobalt ranges from 0.5 to 20 weight % of the total composition; b. wherein the aluminum ranges from 0.1 to 15 weight % of the total composition; c. wherein each of the molybdenum, niobium, copper, titanium, tungsten or vanadium ranges from 0.2 to 10 weight % of the total composition; d. wherein the silicon ranges from 0.01 to 10 weight % of the total composition; e. wherein the nitrogen ranges from 0.01 to 3.0 weight % of the total composition; f. wherein the boron ranges from 0.001 to 0.1 weight % of the total composition; and g. wherein each of the zirconium or hafnium ranges from 0.2 to 6 weight % of the total composition.
 22. The ferrous based component of claim 12, wherein the composition comprises of: a. from 8 to 30 weight % manganese, from 11 to 30 weight % chromium; b. one or more of the alloying additions including, from 0.10 to 1.5 weight % carbon, from 0.10 to 1.5 weight % nitrogen, and their combination thereof; c. from 0.5 to 10 weight % silicon, from 1.0 to 15 weight % aluminum; and d. the balance iron. 